Silicone rubbers and elastomers have been known and used since at least the 1940s. See, for example, U.S. Pat. No. 2,431,878, issued Dec. 2, 1947 to McGregor et al. One useful product is a combination of a silicone elastomer with a filling agent to give a bouncing putty. Silicone-containing polymers known as "bouncing putties" are well described in the patent literature, beginning with U.S. Pat. No. 2,541,851, issued Feb. 13, 1951 to Wright.
Bouncing putties are useful in several applications: (1) as novelty item such as Silly Putty.RTM.; (2) as a hand strength exercise aid for patients with weak hands, for example for stroke victims or for persons recovering from hand surgery; (3) as a hand strength exercise aid for athletes and executives; and (4) as a "worry bead" type product.
Bouncing putties as an exercise aid are sold in a variety of resistances by North Coast Medical Company (San Jose, Calif.) and by others. Bouncing putties can be prepared in flow resistances from very compliant to very resistant. The more resistant putties can provide a useful exercise aid for athletes, such as golfers, who wish to strengthen their grip, or as a novelty item for people with excess nervous energy who often need to be working something with their hands.
One useful putty base is a dimethylboropolysiloxane compound or borosilicone rubber, often a combination of boric acid with silols and reinforcing silicas. One such bouncing putty base is Q2 3233 from Dow Corning.
Workers in the art recognized as early as the 40s that addition of a silica filler enhanced bouncing properties. Fumed silica or ground silica are still useful fillers. Addition of ground silica gives a product like Silly Putty.RTM.. A typical blended putty of this type has an inherent density of about 1.3. Densities of less than about 1.0 g/cc have been difficult to achieve using previous formulations.
Use of alternate fillers, thickening or thinning agents and other materials allow control of a wide variety of properties. For example, a typical modem putty includes oleic acid as a softener and glycerine as a bounce enhancer.
A typical thinning oil is simply silicone oil with a low viscosity, e.g. a viscosity of 10 centistokes (cs). Varying the amount of thinning oil can modify the consistency of a bouncing putty from soft to rock hardness. The intrinsic density of the mixture may vary between about 0.95 (soft) and 1.6 (very hard).
A bouncing putty should provide several desired properties, including good bounce properties and a good "hand," that is, the putty should not be sticky, oily, overly drying, should not leave a residue and should have a long working life.
Others have attempted to achieve light weight by adding lighter fillers but products have suffered from stickiness or oiliness. Previous attempts to incorporate glass or ceramic microspheres have been unsuccessful. The present invention overcomes these problems while providing the advantages of a low density putty.